Chlorine dioxide is a desirable product applied diversely such as in formulating disinfectants and manufacturing paper products. Historically ClO.sub.2 has been prepared commercially by a reaction between a metal chlorate in aqueous solution, such as sodium chlorate, and a relatively strong acid such as sulfuric, phosphoric or hydrochloric acid.
Examples of ClO.sub.2 processes utilizing H.sub.2 SO.sub.4 are shown in U.S. Pat. Nos. 4,081,520; 4,079,123; 3,933,988; and 3,864,456. Examples of ClO.sub.2 processes utilizing HCl are shown in U.S. Pat. Nos. 4,079,123; 4,075,308; 3,933,987; 4,105,751; 3,929,974; and 3,920,801. A process for ClO.sub.2 utilizing phosphoric acid is shown and described in U.S. Pat. No. 4,079,123.
Generally, these present processes for generating ClO.sub.2 utilize an alkali metal chlorate containing feedstock, usually NaClO.sub.3, that also includes a halide salt of the alkali metal. Sodium chlorate feedstock for such a ClO.sub.2 process typically is generated by electrolysis of sodium chloride brine in any well-known manner. Spent brine typically accompanies sodium chlorate withdrawn from the electrolysis cells for use in an accompanying ClO.sub.2 process.
In present ClO.sub.2 processes, the mixture of brine and chlorate is generally fed to one or more reactors where the feedstock contacts a desired acid and reacts to form ClO.sub.2. In these processes, a competing reaction occurs between the metal halide salt and the acid, producing Cl.sub.2. The Cl.sub.2 must be separated from the ClO.sub.2 being generated. Frequently the Cl.sub.2 is reacted to form metal chloride salt or HCl and is then recycled.
In some ClO.sub.2 generation schemes, an additional reducing agent, such as SO.sub.2 or methanol, is added to the mixture of the metal chlorate compound and acid. Yet for some such agents, like SO.sub.2, the relative amount added must be carefully controlled. It has been reported that an excessive quantity of SO.sub.2 causes evolution of significant additional Cl.sub.2 at the expense of ClO.sub.2 production. However, it is suggested that these reducing agents can reduce the evolution of Cl.sub.2 when used in proper proportion.
The reaction between, for example, NaClO.sub.3 and sulfuric acid is known to occur at ambient temperatures. This reaction at moderate temperatures, however, is slow and is therefore unacceptable in a commercial setting. One common method for elevating the reaction rate is to contact the reactants at an elevated temperature, usually between 40.degree. C. and the boiling point of the particular reactant mixture being utilized. Often reduced pressure in the reactor is employed. Reduced pressure has been reported to have a beneficial impact upon the reaction rate, while lowering the boiling point of the reaction mass providing steam for diluting the ClO.sub.2 product.
An elevated concentration of gaseous ClO.sub.2 poses a serious safety risk. Generally between 10 and 15 percent is considered the maximum concentration desirable when handling gaseous ClO.sub.2. It appears that the safe concentration declines as temperature is elevated. A variety of substances are known for diluting ClO.sub.2 as it is produced, including air, steam, and chlorine.
One drawback common to present ClO.sub.2 generation schemes is that the chlorate in an aqueous solution reacted with the acid is valuable. The chlorate must be herefore consumed substantially completely in order for the process to be economical. Since the rate of reaction of the metal chlorate with the acid is strongly a function of the concentration of each, it may be seen that significant reactor residence time can be required to satisfactorily exhaust a given volume of reactants of its metal chlorate content or a substantial quantity of spent reactants must be either recycled or disposed of. Catalysts, functioning to elevate the rate of reaction, could alleviate the impact of low reaction rates associated with operation to very low residual chlorate levels in the reaction mass.
Beyond the addition of reducing agents such as SO.sub.2 or methanol, catilization of the chlorate-acid reaction has not been extensively developed. Vanadium pentoxide, silver, arsenic, manganese, and hexavalent chrome have been suggested as catalyst candidates in U.S. Pat. No. 3,563,702. It is suggested that these catalysts can reduce Cl.sub.2 evolution from the competing reaction of the metal halide with the acid.
Electrolysis of a solution of a metal chlorate and a desired acid potentially offers a useful reaction rate improvement, particularly when processing to very low chlorate levels in the reactant solution. Electrodes utilized in such an electrolysis process would be exposed to a potentially damaging, strongly acidic environment. Therefore, development of a low overvoltage, long-lived electrode would appear essential to development of a commercially useful electolytic ClO.sub.2 process using an acid and a chlorate for feedstock material. Use of electrolysis for ClO.sub.2 generation does not appear to be substantially suggested or developed in prior patented art.
Electrocatalytic anode coatings for use in electrolytic chlorate or chlorine generating cells are known. Some of these coatings contain platinum group metals such as ruthenium or mixtures of platinum group metals and valve metals such as titanium. Typical chlorine or chlorate producing anode coatings are shown in U.S. Pat. Nos. 3,751,296; 3,649,485; 3,770,613; 3,788,968; 3,055,840; and 3,732,157. Use of such coatings upon cathodes for the generation of ClO.sub.2 is not suggested.
There does not appear to be substantial development in the prior art of a relatively limited selection of platinum group metal combinations effective as either a catalyst for the generation of ClO.sub.2 from a metal chlorate and an acid or as an electrocatalyst for the electrocatalytic generation of ClO.sub.2 from the metal chlorate, and the acid has not been suggested.